Download - Week 4
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Prepared by:Mrs Faraziehan Senusi
PA-A11-7C
David P. White Prentice Hall ©
2003
Autoionization
Strong acids and bases
Weak acids and bases
Chapter 2Acids and Bases
Acid-base titration
Acids and Bases
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Lesson Plan
At the end of this topic, the students will be able:
To explain the concept of acids and bases according to Arrhenius, Bronsted and Lewis
To calculate pH, pOH, pKa, pKb and pKw
David P. White Prentice Hall ©
2003
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Arrhenius Theory
• Acid = substance that contains hydrogen and ionize in water to produce hydrogen ion (H+).
HCl H+ + Cl-
• Base = substance that contains the OH (hydroxyl) group and ionize in water to produce hydroxide ion (OH-)
NaOH Na+ + OH-
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Lewis Theory
• A more general definition• Acid = Substance which can accepts a pair
of electrons.• Base = Substance which can donates a pair
of electrons.• Example: BF3 (acid)and NH3 (base)
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Bronsted Theory
• Acid = substance capable of donating a proton (H+).• Base = substance that can accept a proton (H+)
acceptor.• For example:
HF (aq) + H2O (l) H3O+ (aq) + F- (aq)
Base (B)
Acid (B)
conj. acidacidbase conj. base
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For example:
NH3 (aq) + H2O (l) NH4+ (aq) + OH- (aq)
Base (B)
Acid (B)
Bronsted Theory
conj. acidacidbase conj. base
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• Water can act both as Bronsted acid and Bronsted base.
• Water is an amphiprotic.
• Amphiprotic = a substance that can act both as Bronsted acid and Bronsted base.
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Proton Transfer Equilibria in Water
• Proton transfer between acids and bases is FAST IN BOTH DIRECTIONS.
• So, previous equations give:
HF (g) + H2O (l) H3O+ (aq) + F- (aq)
NH3 (aq) + H2O (l) NH4+ (aq) + OH- (aq)
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Conjugate Acids and Conjugate Bases
• When a species donates a proton (so it acts a Bronsted acid), it becomes the conjugate base.
• When a species accepts a proton (so it acts a Bronsted base), it becomes the conjugate acid.
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• A Brønsted-Lowry acid-base reaction occurs when an acid and a base react to form their conjugate base and conjugate acid, respectively:
David P. White Prentice Hall ©
2003
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The Strength of Bronsted Acids and Bronsted Base
The stronger the base, the weaker is its conjugate acid.
Theodore L. Brown , H. Eugene Lemay , Bruce E. Bursten , Catherine J. Murphy ,David P. White, Chemistry the central science.
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• The strength of a Bronsted acid is measured by its acidity constant (Ka).
• The strength of a Bronsted base is measured by its basicity constant (Kb).
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Acidity constant (or acid ionization constant), Ka:
HX (aq) + H2O (l) H3O+ (aq) + X- (aq)
HX
X OH3
aK
• If Ka is very small (Ka <<1) , [HX] is large with respect to [X-],
• So,
1. PROTON RETENTION by the acid is favoured.
2. Very small fraction of acid is DEPROTONATED.
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Basicity constant (or base ionization constant), Kb:
B (aq) + H2O (l) BH+ (aq) + OH- (aq)
B
OH BH
bK
• If Kb is very small (Kb <<1) , [B] is large with respect to [BH+],
• So,
1. Very small fraction of base is PROTONATED.
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Autopyrolysis (or Autoionization)
• Since water is amphiprotic, a proton transfer equilibrium exists with an absence of added acids or bases.
• In pure water at 25oC,
[H3O+] = [OH-] = 1.0 x 10-7 M
Kw =1.0 x 10-14
• Autopyrolysis constant:
Kw = [H3O+][OH-]
2H2O(l) H3O+(aq) + OH-(aq)
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OHOHO2H
OHHAOHA
AOHOHHA
32
2
32
Relation between Ka and Kb of conjugate acid-base pair
• The sum of two dissociation reactions is the autoionization of water.
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• Express strength of acid in terms of the strength of its conjugate base
Ka.Kb = Kw • Two reactions(c.a–base pairs) added to give
a third reaction, the equilibrium constant for the third reaction is the product of the equilibrium constants for the two added reactions
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• pK= -log K• pOH = - log[OH-]• pH = - log[H+]• pKa + pKb = pKw• pH + pOH = 14.00
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Strong and Weak Acids and Bases
• An acid or base is classified as either weak or strong depending on the size of its acidity constant.
• Strong acid: The proton transfer equilibrium lies strongly in favour of donation of a proton to water.
pKa < 0, Ka > 1• Weak acid: The proton transfer equilibrium lies in
favour of ionized acid.
pKa > 0, Ka < 1
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• Strong base: species that is mostly protonated in water.
pKb < 0, Kb > 1
• Weak base: species that is only partially protonated in water.
pKb > 0, Kb < 1
Strong and Weak Acids and Bases
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Example Strong Acid & Strong Base
David P. White Prentice Hall ©
2003
?pH
10 1.0H
10 1.0 10 1.0 0 f
10 1.0 10 1.0 10 1.0 -
0 0 10 1.0 i
(aq) Cl (aq) H (aq) HCl
3-
3-3-
3-3-3-
3-
-
a)
b) ?
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a) Calculate the pH of a 0.50 M HF solution at 25oC, Ka =7.1 x 10-4
b) Calculate the pH of a 0.050 M HF solution at 25oC, Ka =7.1 x 10-4
• Require x to be less than 5%. approximation is valid if expression is equal to or less than 5%
a
acbbx
2
42
Example Weak Acid
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• Write for Ka = _______ = 7.1 x 10-4 •
• Write for ionization constant expression,Ka =
• Then, x must be small compared to 0.50, we make approximation of
• Calculate for pH,
pH = ?b) Try calculate the pH of a 0.050 M HF ??? ~ Quadratic equation
a)
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David P. White Prentice Hall ©
2003
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Percent Ionization
• Measure the strength of an acid
• The stronger the acid, the greater the percent ionization
%100ionconcentrat acid initial
mequilibriuat ion concentrat acid ionized
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• Calculate the pH of a 0.40 M ammonia solution. Kb =1.8 x 10-5.
Example Weak Base
Kb = ___________ = 1.8 x 10-5
x = ?
Check % of approximation = ≤ 5% ?
pOH = ? pH = ?
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• The conjugate base of any strong acid is a weak base.
• The conjugate base of any weak acid is a strong base.
• The conjugate acid of any strong base is a weak acid.
• The conjugate acid of any weak base is a strong acid.
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Polyprotic Acids• Polyprotic acid = a species that loses
protons in succession, and successive deprotonations are progressively less favourable.
• This is because the additional electrostatic work must be done to remove the positively charged proton.
• These substances may yield more than one hydrogen ion per molecule.
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• For example, for carbonic acid, H2CO3,
• Note that the conjugate base in the first ionization stage becomes the acid in the second ionization stage.
David P. White Prentice Hall ©
2003
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Characteristic of Bronsted Acid
• Acidic proton is the donatable proton.• There are 3 types of acidic proton:
1. Aqua Acid = the acidic proton is on a water molecule coordinated to a central metal ion.
• For example:
[Fe(H2O)6]3+(aq) + H2O(l) Fe(H2O)5(OH)]2+(aq) + H3O+(aq)
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2. Hydroxoacid = the acidic proton is on a hydroxyl group without a neighbouring oxo group (=O)
• For example: Te(OH)6
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3. Oxoacid = the acidic proton is on a hydroxyl group with an oxo group (=O) attached to the same atom.
• For example: H2SO4
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• The successive stages in the deprotonation of an aqua acid:
AQUA ACID HYDROXOACID OXOACID
• The strength of aqua acids increase with increasing positive charge of the central metal ion and with decreasing ionic radius.
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David P. White Prentice Hall ©
2003
• Oxalic acid (H2C2O4) is a poisonous substance used chiefly as a bleaching and cleansing agent (for example, to remove bathtub rings).
• Calculate the concentrations of all the species present at equilibrium in a 0.10 M solution.
Ka1 = 6.5 x10-2 & Ka2 = 6.1 x10-5
Example Polyprotic Acids
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Ka1 = ___________ = 6.5 x10-2
x = ?
Check % of approximation = ≤ 5% ? If not, ~ quadratic equation
At equilibrium,
First stage of ionization:
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Second stage of ionization:
Ka2 = ___________ = 6.1 x10-5
y = ?
Check % of approximation = ≤ 5% ? If not, ~ quadratic equation
At equilibrium,